Laser flashlight

ABSTRACT

A laser flashlight employs an emitter disposed within a housing for emitting a coherent light beam having a gaussian spatial profile along an optical axis toward the exit face of the housing. An optical system disposed within the housing intermediate the emitter and the exit face of the housing includes a laser element pumped by the emitter, a frequency/wavelength converter, and a resonator, to form the coherent light into a laser beam. A beam expander receives the laser beam, disperses the laser beam, and transmits the dispersed laser beam from the light emitting end of the housing into the ambient environment.

PRIORITY CLAIM

[0001] This application is a continuation of U.S. patent applicationSer. No. 10/162,033, filed Jul. 26, 2002, which is a divisional of U.S.patent application Ser. No. 09/706,911 filed Nov. 7, 2000 and issued asU.S. Pat. No. 6,431,732, which is a continuation of U.S. patentapplication Ser. No. 09/111,335, filed Jul. 7, 1998 and issued as U.S.Pat. No. 6,142,650, which claimed priority from U.S. Provisional PatentApplication Serial No. 60/052,159, filed Jul. 10, 1997.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to the field of portableillumination devices for illuminating an ambient environment. Morespecifically, the present invention is directed to a hand-held laserflashlight which illuminates an ambient environment while minimizing therisk of causing irreversible eye damage.

[0003] Law enforcement, corrections, and military personnel faceincreasingly greater threats in their daily activities. Routine trafficstops can end in officers finding themselves in life-threateningsituations. Domestic disputes and drug enforcement activities areamongst the most dangerous law enforcement personnel face. Also,increasingly law enforcement personnel face situations such as riots orunruly groups of individuals where certain lethal options cannot be usedor would only serve to further exacerbate the situation. Theavailability of non-lethal weapons expands the range of optionsavailable to law enforcement in reacting to potentially violent orlife-threatening situations.

[0004] Hand-held flashlights have been in widespread usage in many areasfor many years. For example, in the area of forensics, law enforcementpersonnel have found flashlights to be so useful that incandescentflashlights have become “standard issue.” These “standard issue”flashlights produce a bright beam from a small and relativelylight-weight package. Such “standard issue” flashlights have, interalia, been used by law enforcement personnel to illuminate crime scenes,to disorient and confuse suspects and to physically subdue suspects Theeffect of laser light on human eyesight can be separated into threecategories: glare effects, dazzling or flashblinding, and permanentdamage. The retinal damage threshold in general depends upon the laserwavelength, the exposure duration, and whether or not the laser isoperated in continuous wave or pulsed or modulated mode and, if pulsed,the repetition rate and the pulse duration. If the laser light intensityis such that the damage threshold for the eye is exceeded, lesions areproduced that are permanent. This damage occurs at the location of theretina for visible and near-infrared light. Ultraviolet and far-infraredlight on the other hand are absorbed in the cornea and the light neverreaches the retina.

[0005] The exact intensities where glare stops and dazzling occurs aredifficult to define with precision and depend to a large extent on theindividual involved. In general, however, glare occurs first and resultsin little or no loss of vision performance. In fact, after a lasersource is turned off, in the glare region no after-effects or latentimages occur. If the intensity is increased, however, at some intensityor over a range of intensities, dazzling occurs. If the laser is turnedoff, short-term partial loss of vision occurs, typically lasting forseconds or tens of seconds. At still greater intensities theafter-effect is substantially longer, perhaps as long as-minutes. Thiseffect is best compared to the use of a fundus camera to photograph theretina of the human eye, usually under fully dilated conditions.Substantial vision loss can occur and last for many minutes.

[0006] Some inroads have also been made in applying laser technology toportable illumination devices in limited areas. One major drawback ofsuch uses of lasers has been that the laser beam emitted by suchlow-power devices has the potential to produce irreversible eye damageif a person gazes directly into the light source. Thus, these devicesare not “eyesafe.” Naturally, this problem becomes exacerbated asattempts are made to increase the output power of such devices. Anothersignificant limitation associated with such portable laser emittingdevices is that they have, to date, been unable to produce nearly asmuch light as comparably sized incandescent flashlights. Accordingly,their lack of versatility and overall poor performance has limited theiruse.

[0007] The desirability of producing glare or flashblind effects,whereby a temporary reduction to visual performance results fromexposure to laser light, has been disclosed, among others, by German inU.S. Pat. No. 5,685,636. However, the laser flashlight device asdescribed by German suffers from a number of critical deficiencies,especially with regard to the laser safety aspects associated with itsintended use as a portable visual security device against mobiletargets. For example, the eye safety of the radiation produced with theportable laser security device of German, could only be assured beyond acertain range, which, at the minimum, is set at 3 m. Thus, to keep theintensity below 58.3 mW/cm², the upper limit recited as corresponding topermanent undesirable damage to the eye, would require spot sizediameters of at least 5 cm for power levels beyond 1 W. Since the damagethreshold decreases as the exposure time increases (FIG. 1 of U.S. Pat.No. 5,685,636) the operational range of power must be reduced and/or thebeam spread parameter increased to assure safety at exposure times whichmay be longer than 100 msec.

[0008] German teaches that the spot size is to be adjusted using amovable collimating lens contained within the apparatus, with theexplicit purpose of reducing the laser beam spread produced by asemiconductor laser with a highly divergent beam. Clearly, such manualadjustments may be difficult to realize in real life situations whererapidly moving intruders are encountered. It will require the operatorto guess the likely range of the target while taking additional care toascertain that the range always exceed a minimum value even as theexposure duration is kept short enough to avoid permanent eye damage,yet long enough to produce the desired deterrent effect. These areclearly difficult conditions to fulfil in high stress situations whererapid response times are essential.

[0009] In terms of the effectiveness of the laser flashlight in use as anon-lethal security device, the embodiments and methods of operation astaught by German again fall short. In particular, whereas it wasappreciated that shorter wavelengths are more effective in producing thedesired glare and flashblind effects, requiring less power than a redsemiconductor laser, no disclosure was provided with regard to eithermethods and/or structures for producing said shorter wavelengths laserdevices, in particular, at the green wavelengths recognized asespecially effective for this application.

[0010] It may be noted that the specific example provided, namely thelaser produced by Santa Fe lasers, is neither compact enough to beprovided within a standard flashlight package, nor is it capable ofproducing the desired eye safety features. In particular, it mustfurther be noted that with regard to the green radiation produced byfrequency doubled, diode pumped solid state laser, it is well known inthe art that the beam properties from such a laser are very differentthan those produced by typical diode lasers. For example, the laserradiation of a solid state laser tends to be much less divergent, havinga higher degree of spatial coherence than the output of a diode laser.Consequently the collimating lens described by German as the criticalelement for reducing the spread of a laser beam is entirely incompatiblewith the near diffraction limited radiation produced from mostcrystalline solid state laser. In fact, use of such a short focal lengthlens may result in drastic focusing effects leading to smaller beam spotsizes and therefore much higher intensities even at the longest rangescited by German, thereby severely compromising eye safety. It wasclearly not realized by German that use of a movable collimating orfocusing lens as a means for adjusting the beam spread of the laserdevice is insufficient to assure eye safety at arbitrary ranges from thesolid state laser device. Although other optical elements may beenvisioned that may be capable of increasing rather than decreasing thebeam spread, no such elements were described in the German patent, yetthe choice of a specific such optical element represents an essentialdesign feature for the security applications contemplated.

[0011] Alternative prior art that directed to portable laserapplications also fail to fulfil the requirements of a safe andeffective laser flashlight. For example, U.S. Pat. No. 5,396,069 toCraig et al describes a packaged solid state laser device for use as aportable night vision apparatus. The laser disclosed therein does notinclude beam shaping or other optics necessary to assure eye safety atall power levels. In particular Craig teaches that his devicespecifically requires eye safety warning labels, and that such device isrecognized not to be eye safe when used with the recommended lightsource, in this case a diode laser of power up to 50 mW. Craig alsoteaches a variable focus adjustment system, which is at the operator'sdisposal. Clearly, such adjustments will be difficult to executemanually and still provide an output intensity that is an effectivedeterrent against a moving target, while also assuring safety accordingto accepted ANSI standard for a given exposure duration. Furthermore,none of the embodiments shown conform to a standard flashlight packagingconstraints imposed herein. Other compact solid state laser have beendescribed in the literature, but none that would have the necessary eyesafety features at sufficiently high power levels to provide onlyreversible flashblinding or glare effects.

[0012] It should be readily appreciated that to guarantee eye safetyunder all conditions, a laser flashlight used as a security deviceagainst moving human targets must be eye safe at the aperture, or rightat the exit face of the flashlight device. Only then can the operator beassured that the laser radiation would not produce permanent, lethal eyedamage at any range, for a given set of power settings, selected toprovide the desired effects. Yet, without such assurance, the likelihoodof use of said flashlight by law enforcement personnel is not very high.None of the laser technologies or commercially available devices knownin the current art include the requisite fail safe mechanism forassuring both automatic safety and effectiveness.

SUMMARY OF THE INVENTION

[0013] Briefly stated, the invention in a preferred form is a laserflashlight for emitting an eyesafe light dispersion pattern into theeyes of a person. The laser flashlight employs an emitter disposedwithin the flashlight housing for emitting a coherent light beam alongan optical axis toward the exit face of the housing. An optical system,disposed along the optical axis intermediate the emitter and the exitface, is configured to provide an output laser beam having a spatialprofile that is substantially gaussian. A light transmissive beamexpander, disposed along the optical axis proximate to the exit face ofthe housing, is configured to limit the intensity of the output laserbeam at the eyes of the person, for any power of the emitter, up to andincluding the maximum power of the emitter, to a level which is alwaysless than the ANSI safety standard adapted for the wavelength of theoutput laser beam.

[0014] In different embodiments of the laser flashlight of the presentinvention, the emitter and the optical path configuration can be eithera diode laser optically connected to an optical fiber or, alternatively,a diode laser or laser array pumping a solid state crystal disposedwithin an optical resonator. The optical system may also include one ormore lenses to focus the coherent light emitted by the emitter on alaser crystal. Thus, the emitter can be directly coupled to the beamexpander via an optical fiber or coupled therewith using a resonator.Further, the laser beam emitted by the flashlight may be eithercontinuous wave or pulsed.

[0015] The laser resonator comprising the optical system of theflashlight may further include a harmonic generating crystal to shiftthe wavelength of the laser beam to either a longer or shorter spectralrange. In alternative embodiments, the harmonic generating crystal maybe a nonlinear or a linear wavelength shifting element. Preferably, andoptimally when the flashlight is used as a security device, theharmonically generated light has a wavelength in the visible range,close to the green portion of the spectrum where light sensitivity ofthe eye is at a maximum. In such a laser flashlight, the optical systempreferably includes optical coatings designed to reverse the directionof the harmonic light which is traveling back toward the emitter tothereby increase the efficiency of the flashlight.

[0016] It is, accordingly, an object of the present invention to providean eyesafe laser flashlight which will not produce irreversible eyedamage if a person gazes directly into the flashlight.

[0017] It is a further object of the present invention to provide aportable laser flashlight having the general size, shape and weightcharacteristics of a “standard issue” incandescent flashlight used bylaw enforcement personnel.

[0018] It is still another object of the present invention to provide ahand-held laser flashlight which is capable of emitting significantlymore light than prior hand-held laser-based illumination devices tothereby enable the laser flashlight to be effectively used over longerdistances and/or to illuminate larger areas.

[0019] It is yet another object of the present invention to provide aportable laser flashlight having at least one laser emitter that can bemodulated in either periodic or random fashion to more efficientlyproduce a bright beam of light.

[0020] It is an additional object of the present invention to provide aneyesafe portable laser flashlight which employs at least one laseremitter and is capable of operating at more than one output wavelength.

[0021] It is still another object of the present invention to provide ahand-held laser flashlight which provides an optimal combination of (a)simplicity; (b) reliability; (c) durability; (d) versatility; and (e)efficiency.

[0022] Numerous other advantages and features of the present inventionwill become apparent to those of ordinary skill in the art from thefollowing detailed description of the invention, from the claims andfrom the accompanying drawings.

[0023] It is still another object to of the present invention to providea portable laser flashlight including the feature of constant brightnessover a range of power settings. This feature enables variation of thepower settings without affecting the beam spread, thereby assuring eyesafety even at the highest power settings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The preferred embodiments of the present invention will bedescribed below with reference to the accompanying Figures wherein likenumerals represent like structures and wherein:

[0025]FIG. 1 is a cross-sectional side elevation view of a firstpreferred embodiment of the laser flashlight of the present invention;

[0026]FIG. 2 is a chart illustrating the laser intensity at the eyes ofa subject as a function of pulse duration required to achieve either theglare and flashblinding region or the eye damage region in accordancewith an accepted laser safety standard;

[0027]FIG. 3 is a chart illustrating the output power of the laserflashlight of FIG. 1 as a function of diode output power;

[0028]FIG. 4 is a cross-sectional elevation view of another preferredembodiment of the laser flashlight of the present invention;

[0029]FIG. 5 is a schematic representation of a first embodiment of agreen light solid state laser and diode for use with the presentinvention;

[0030]FIG. 6 is a schematic representation of a second embodiment of agreen light solid state laser and diode for use with the presentinvention;

[0031]FIG. 7 is a schematic representation of a third embodiment of agreen light solid state laser and diode for use with the presentinvention;

[0032]FIG. 8 is a schematic representation of a fourth embodiment of agreen light solid state laser and diode for use with the presentinvention;

[0033]FIG. 9 is a schematic representation of dual-color laser anddiodes for use with the present invention; and

[0034]FIG. 10 is a front view of an exemplary pulse wheel which may beelectronically incorporated into the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] A laser flashlight 10 in accordance with a first preferredembodiment of the present invention will be described below withreference to FIG. 1. As shown therein, laser flashlight 10 generallyincludes a housing 12, a power supply 14, a diode source, or package 16,a resonator 18 and a beam expander 20. As shown, these components arepreferably axially aligned along an axis A which is defined by thedirection of travel of laser beams emitted from diode package 16.Housing 12 comprises a generally cylindrical main body coaxiallydisposed about axis A, a first end 22, a second light emitting end oraperture 24, a lens-retaining shell 26 disposed at the second end 24 andan end cap 28 disposed at the first end 22.

[0036] A battery pack 30 having a plurality of batteries 32 is disposedwithin the housing 12 to supply electricity to a variety of components.In particular, battery pack 30 preferably includes six 1.2 volt 3 ampnickel metal hydride rechargeable batteries 32 commercially availablefrom Duracell Corporation in size 4/3A with Model No. DH43289. Two ofthe six batteries 32 are connected in series to drive diode package 16.The remaining four batteries 32 are connected to supply electricity tothe electronics associated with the power supply 14 as well as to thethermo-electric cooling (TEC) unit 34 used to cool the diode package 16.Battery pack 30 should yield approximately two hours of operating timedepending on the particular diode package 16 and TEC unit 34 employed.

[0037] Electricity is applied to the diode package 16 and the TEC unit34 from battery pack 30 via the power supply 14. Power supply 14preferably includes electrical conductors as known in the art and anon/off switch 36 for selectively interrupting the flow of electricityfrom battery pack 30. To modulate the production of laser beams by thediode package 16 and to regulate the TEC unit 34, the power supply 14can include microprocessor controlled electronics as desired. A standbyand enable switch 38 is also preferably electrically connected to thebattery pack 30 such that the on/off switch 36 is inoperable unless theenable switch 38 is placed in a “standby” position. Thus, the operatormust place the laser flashlight 10 into a “standby” mode before it canbe turned on.

[0038] As noted above, the power supply electronics can be used toeither periodically or randomly interrupt the flow of electricity frombattery pack 30 to diode package 16. For example, the power supply 14can include electronics which will periodically pulse electricity frombattery pack 30 to diode package 16 at a rate of 20 hertz with a 50%duty cycle to create a strobe-like effect. Naturally, these electronicsare also be powered by battery pack 30 and the frequency and duty cyclecan be varied as desired. For example, with reference to FIG. 10, theelectronics may pulse the diode package at a pulse sequence which isequivalent to a mechanical wheel 40 used to mechanically convert acontinuous wave laser beam to a pulsed beam. The wheel 40 may beconceptually envisioned as having notches 42 in the outer perimeter 44which define a series of radially extending tabs 46. The pulse wheel 40is positioned such that the outer perimeter 44 of the wheel 40 is in thepath of a light beam. It should be appreciated that the light beam isalternately blocked and then allowed to pass as the pulse wheel 40rotates and the tabs 46 and notches 42 pass through the light beam. Inthe preferred embodiment, the arc width of tabs 46 and notches 44 areselected to produce an intermittent asymmetric pulse. The pulse wheel 40shown in FIG. 10 has a 50% duty cycle and produces a 20 cycle flicker at120 RPM. Experimentation has shown that such a flicker sequence isextremely disorienting to most people.

[0039] The diode package 16 is disposed within the cylindrical bodyalong axis A such that laser beams emitted therefrom will be directedtowards the aperture 24 of housing 12. Naturally, the power supply 14and the battery pack 30 are disposed rearwardly of diode package, i.e.,towards first end 22 of housing 12, so as not to optically interferewith laser beams produced by the diode package 16. A diode 48 whichoperates at 808 nanometers (nm) and which produces approximately one tofour watts of power is preferably used. Diode package 16 includes ahigh-heat-load (HHL) package that is moisture sealed and contains TECunit 34 for adjusting the diode operating temperature and, thus, itswavelength to match the absorption of the laser crystal. However, avariety of other diode packages could be utilized with the presentinvention. While the diode package 16 is preferably supplied electricityfrom battery pack 30 on a continuous basis when laser flashlight 10 isin an “on” condition, the diode package 16 could be pulsed atappropriate rates as desired to prolong the operating life of batteriesand/or produce pulsating effects as desired.

[0040] Laser beams exiting the diode package 16 are focused into thelaser crystal of the optical system 50 to excite laser emission from theresonator 18. The beam expander 20 preferably includes a plurality oflight transmissive lenses 52, 54, 56, 58 to disperse the laser beamdelivered thereto by the resonator 18, whereby laser flashlight 10 emitsan eyesafe light dispersion pattern into the ambient environment at theaperture 24. Lenses 54, 56, 58 may be aspheric. Outer lens 52 is a flatelement which preferably ensures a tight seal with shell 26 and does notoptically distort light passing therethrough. Lenses 54 and 56 arepreferably concave-convex to ensure a wide dispersion pattern of lightpassing therethrough. Additionally, lens 58 is preferably concave, bothin the direction of and in the direction opposite to the travel of lightthe rethrough (i.e. double concave) to, once again, maximize lightdispersion. As shown in FIG. 1, lenses 52, 54, 56, 58 are axiallydisposed about axis A. Lenses 52, 54, 56, 58 function as a group todisperse laser beams transmitted from the resonator across the first endto ensure compliance with the Food and Drug Administration CDRHrequirements for eye safety. Accordingly, the laser beam emitted fromthe front surface 60 of lens 52 is eyesafe throughout its extant.

[0041] The best guidance in matters pertaining to damage in the humaneye from laser sources is provided by the American National StandardsInstitute (ANSI) laser safety standard ANSI Z136.1-1993. The retinaldamage threshold in general depends upon the laser wavelength, theexposure duration, and whether or not the laser is operated incontinuous wave or pulsed or modulated mode and, if pulse operated, therepetition rate and the pulse duration. Damage thresholds and maximumpermissible exposure (MPE) are usually calculated for a dark-adapted ora fully dilated eye whose pupil is 7 mm in diameter. Lasers of the typeused in the subject invention are continues wave or modulated. To assessthe eye-safety issue for these lasers, it is necessary to calculate theintensity (W or mW/cm²) at the location of the eye and the exposureduration.

[0042]FIG. 2 shows the MPE and ten times the MPE, for the range of blueand green wavelengths (400-550 nm) as a function of pulse or exposureduration according to ANSI Z136.1-193. It can be seen that the damagethreshold decreases as exposure duration increases. The glare regionoccurs below the ANSI standard curve whereas flashblinding or dazzlingoccurs in the upper region near the ANSI curve and between the ANSIcurve and the 10 times or damage threshold curve. For a 10 msec (0.01)second exposure, approximately the minimum needed to produce glare orflashblinding, the damage threshold is approximately 56.9 mW/cm² (0.0569W/cm²). For a 0.25 sec exposure, which characterizes the typical blinkresponse in humans, the damage threshold is approximately 26.0 mW/cm².The ANSI standard for these two cases gives 5.69 and 2.60 mW/cm²,respectively.

[0043] The laser flashlight 10 can be used to induce a number ofaversion responses in subjects or individuals that can provide asubstantial advantage to the user. One aversion response is for thesubject to immediately turn away from the source of the bright light.This is a substantial advantage for the user, especially in dealing witharmed or dangerous individuals. Another response is for the subject todelay any further action or to hesitate, providing the user with time toassess the appropriate responses. One cause of the delay is because asubject wants to see the source of the bright light but cannot seearound the bright beam. In effect an illuminated individual sees an“optical wall” of light, thus protecting the user. Some illuminatedindividuals experience dizziness or vertigo. This is especially truewhen the laser flashlight is modulated at a random or asymmetric rate.

[0044] The laser flashlight 10 may be used for crime-sceneinvestigation, particularly in connection with identifying the locationof fingerprints. The green laser flashlight operating near 532 nm isparticularly useful since it can be used to cause various Rhodamine orother dyes to fluoresce in the visible region. Such dyes are routinelyused in fingerprint dusting powders. The blue region of the spectrum canalso be used to excite other dyes of interest. A green laser flashlightis more effective in providing illumination in a smoke filledenvironment than an ordinary flashlight. A laser flashlight 10 operatingin the green region near 532 nm is very effective in underwaterapplications since it is near the peak transmission of water, allowing alarger penetration depth than a common flashlight.

[0045] An alternative preferred embodiment of the laser flashlight ofthe instant invention is depicted in FIG. 4. As shown therein, laserflashlight 10′ includes a battery pack 30′, a power supply 14, a beamexpander 20, an optical system 50′ including an optical fiber assembly62 and a diode package 16′. The primary difference between laserflashlight 10′ and laser flashlight 10 is that laser flashlight 10′utilizes a diode laser 64 which is directly coupled to beam expander viaoptical fiber assembly 62 to generate laser beams rather than thediodepumped solid-state laser of the FIG. 1, embodiment. The opticalfiber assembly 62 includes an optic fiber 66 and a fiber optic connector68. Diode lasers 64 of this nature are becoming increasingly availablein higher and higher power ratings for various wavelengths, includingthe green range.

[0046] Since the output of a diode laser 64 can be converted to otherwavelengths via many of the non-linear resonators described below, thelaser flashlight 10′ could, optionally, be equipped with a suitableresonator depending on the diode laser 64 selected. However, this laserflashlight configuration is preferred due to its simplicity and itspower, weight and size minimization capabilities. Diode lasers 64 ofcomparable power and frequency ratings to those discussed above arepreferred for use with this embodiment. Finally, since there is no needto cool optical fiber 66 (i.e., there is preferably no resonator to coolin this embodiment), battery pack 30′ can be modified to utilize stillfewer batteries.

[0047] The various components of the laser flashlights 10, 10′ are, asshown, sized, shaped and oriented such that laser flashlight 10, 10′generally mimics the size, shape, weight and function of a “standardissue” incandescent flashlight used by law enforcement personnel.Accordingly, the hand-held laser flashlight 10, 10′ of the instantinvention can be used as a non-lethal weapon in the manner that“standard issue” incandescent flashlights are so used. Laser flashlights10, 10′ can also be configured in a sealed water-tight manner so that itcan be operated under water and in inclement weather.

[0048] With reference to FIG. 5, the optical system 50 preferablyincludes a sapphire crystal window 70 to dissipate heat, a laser crystal72, and a harmonic generating crystal 74 for shifting the fundamentallasing wavelength of the resonator 18. In the embodiment shown in FIG.5, the resonator 18 first shifts the wavelength of the laser beam from880 nm to an intermediate wavelength of 1064 nm. A second harmonicgenerating crystal 74 shifts the wavelength of the laser beam from thefundamental intermediate wavelength of 1064 nm to 532 nm (green light).To completely utilize the green light, the green light travelingbackwards toward the laser crystal 72 must be turned around and made toco-propagate with the green light traveling toward the outcoupler 76. A“green trapping mirror” or harmonic high reflector 78 may be disposedbetween the laser crystal 72 and the harmonic generating crystal 74.FIG. 3 illustrates the output power attainable by the laser flashlight10 of FIG. 1 as a function of diode output power.

[0049] Other non-linear schemes can also be used such as tripling,quadrupling sum-frequency generation, parametric oscillation, Ramanconversion, etc., to produce a wide variety of different wavelengthsuseful for different purposes. Additionally, laser materials capable ofoperating at different wavelengths can be utilized to provide thefurther flexibility of operation at the various transition pointsoffered thereby. For example, Nd:Vanadate or Nd:YAG laser crystals haveadditional transitions located in the vicinity of 920 nm and 1350 nm.Long term fluctuations in output power can be minimized by utilizing aTEC on the non-linear crystal.

[0050]FIG. 5 illustrates a first embodiment of a green light solid statelaser and diode 80 for use in laser flashlight 10. An optical fiber 82is mated or pigtailed to the diode 80. Typically the optical fiber 82will be only a few centimeters long. The diode 80 can emit up to 4 wattsof continuous wave pump power near 808 nm, for example. The opticalfiber 82, which typically can have 90% transmission, is usually a cladmultimode silica fiber with a core diameter between 100-600 μm and anumerical aperture (NA) of 0.22. For higher power versions of theflashlight 10, the single diode 80 can be replaced by an array or a barof laser diodes, thereby providing fiber output powers of up to 20 wattsor more continuous wave.

[0051] A single lens 84 may be used to collect the diode light and imageit with unity magnification into the Nd:YVO₄ (vanadate) laser crystal72, thereby providing an excitation spot having a diameter which isessentially equal to the fiber core diameter. Alternatively, one or morelenses may be used to produce a spot diameter in the vanadate lasercrystal 72 which is smaller than the fiber core diameter. The smallestspot size that can be achieved is determined by the beam-quality of thediode light emerging from the fiber 82, which is typically multimode andfar from the diffraction-limit. Excitation spots having spot diametersbetween about 50 and 600 μm are easily produced, depending on the diodesource 16 and the focusing system 84. The smallest spot sizes producethe highest laser efficiency.

[0052] The vanadate laser crystal 72 may be only 1-2 mm thick and have aNd doping that is large enough to absorb most or all of the diode light.The vanadate laser crystal 72 is arranged to be in intimate contact witha sapphire window 70 whose purpose is to draw the heat out of thevanadate and transfer it to the heat sink 86 which is preferablyoxygen-free Cu. The thermal conductivity of vanadate is very low (0.05W/cm-K) while that of sapphire is 5.6 times greater (0.280 W/cm-K).Because sapphire is a clear optical material the vanadate laser crystal72 is optically-pumped through the sapphire crystal 70. The predominantthermal gradients with this arrangement are in the longitudinal or axialdirection or the optical axis A of the laser. Consequently, the thermalgradients do not affect beam-propagation and the laser is insensitive tothermal effects over a wide range of input powers. The face-cooled laseris described in greater detail in U.S. patent application Ser. No.08/857,700, filed May 16, 1997 and U.S. patent application Ser. No.09/093,508 filed Jun. 8, 1997, which are hereby incorporated byreference.

[0053] The front face 90 (facing outcoupler 76) and the rear face 96 ofsapphire crystal 70 are each coated 88, 98 with an anti-reflective (AR)material around 808 nm to eliminate reflective losses of resonatorlight. The rear face 92 of the vanadate laser crystal 72 is coated 94with a material that is highly-transmissive (HT) at the diode wavelengthbut which is also a highly-reflective (HR) at the laser transition ofinterest and the harmonic wavelength. In a first example, coating 94 maybe highly-reflective at 1064 nm and 532 nm. In a second example, coating94 may be highly-reflective at 945 nm and 473 nm.

[0054] The outcoupler 76 may have two variations. The first variation ispreferred when it is desirable to have the laser operate at afundamental laser transition, in which case the curved, rear face 100 ofthe outcoupler 76 is coated 102 with a partially-reflective (PR)material at the laser wavelength. The second variation is preferred whenit is desirable to use an intra-cavity nonlinear crystal like that shownin FIG. 5 to frequency double the fundamental laser transition into thevisible region. In that case the curved, rear face 100 of the outcoupler76 is coated 102 with a material which is HR at the fundamentalwavelength and HT at the frequency doubled wavelength. The front, outputface 104 of the outcoupler 76 is coated 106 with a material which is ARat either the fundamental or the second-harmonic wavelength, dependingupon whether the first or second variation is used.

[0055] The outcoupler substrate material is typically fused silica butcan also be any other transmissive optical material such as glass,sapphire, or other. The nonlinear harmonic generation crystal 74 isplaced inside the resonator 18 to convert the laser light from afundamental frequency and wavelength to a harmonic frequency andwavelength. In a preferred embodiment, the harmonic generation crystal74 is a second harmonic generation crystal for converting a fundamentalwavelength of 1064 nm to a harmonic wavelength of 532 nm. The conversionefficiency is highest for intra-cavity operation since the fundamentalintensity inside the resonator 18 is many times higher than what couldbe obtained by operating the laser at the fundamental wavelength andthen focusing the output into an external nonlinear crystal. Theconversion efficiency from the fundamental to the second-harmonic iscritically dependent upon the intensity of the fundamental beam.

[0056] A harmonic high reflector 78 or mirror, for example asecond-harmonic reflector, may be positioned between the laser crystal72 and the harmonic generation crystal 74 to increase the efficiency ofthe flashlight 10. The rear face 108 (facing the diode) of the mirror 78is coated 110 with a material which is AR at the fundamental wavelength.The front face 109 of the mirror 78 is coated 111 with a material whichis AR at the fundamental wavelength and HR at the second-harmonicwavelength. When employed, the mirror 78 collects most of the backwardstraveling second-harmonic light and transmits it coincident with theforward traveling light, thus increasing the conversion efficiency andoverall laser efficiency.

[0057] The fundamental or harmonic beam emerging from the laserresonator 18 is very intense and usually not eye-safe. To make the lasereye-safe, a beam expander 20 functions as an optical group to firstincrease the beam diameter and then collimate it or produce a beamdiverging from the objective lens 112 at a desired divergence. Thediameter of the laser beam impinging the input surface of the expanderis less than the diameter of the laser beam at the exit surface of thebeam expander. Because the beam is always collimated or diverging afterthe objective lens 112, it is always eye-safe after the front surface 60of lens 52 and any distance beyond. This is one of the key designfeatures. The lenses 112 and 114 or 52, 54, 56, 58 of FIG. 1 used forthe beam expander can consist of two or more of conventional ornon-conventional design and may be spherical or aspherical. In onepreferred embodiment, a gradient index (GRIN) or an aspheric lens 114 isused to rapidly expand the beam. Collimation is then achieved by usingthe objective lens 112. Since the GRIN or aspheric lens 114 can bedesigned to have a large NA, the distance between that lens 114 and theobjective lens 112 is minimized, allowing the laser flashlight 10 to beof minimal length. The use of conventional spherical lenses would resultin a much longer beam expander.

[0058] The resonator 18 is a hemispherical type, defined by the flatrear surface 92 of the vanadate laser crystal 72 and the curved rearface 100/104 of the outcoupler 76. An optimal design provides an outputbeam that is a Gaussian or fundamental TEM₀₀ mode. This may be achievedby choosing the resonator spacing and outcoupler curvature so that theminimum waist or spot diameter, located at the rear face 92 of thevanadate laser crystal 72, is approximately matched to the excitationspot diameter of the diode light. The beam diameter inside the resonator18 is minimum at the rear surface 92 of the laser crystal 72.Consequently, the nonlinear harmonic generation crystal 74 is placed asclose as possible to the laser crystal 72 so that the fundamentalintensity is maximized. In practice, it has been found that shortresonators in the range of 1-3 cm and outcoupler curvatures in the rangeof 5-15 cm are optimum for maximizing the laser efficiency and producinga diffraction-limited fundamental output beam.

[0059] It should be appreciated that other extensions of this technologycan be made, for example to incorporate intra-cavity third or fourthharmonic generation or an intra-cavity optical parametric oscillator. Itis also possible to include dispersive elements for tuning the output oftunable solid-state laser materials to different output wavelengths.Lasers of the type shown in FIG. 5 may be operated at more than onewavelength simultaneously.

[0060] The embodiment illustrated in FIG. 6 is similar to that shown inFIG. 5 except that the diode source 116 and optical system 118 fordelivering the diode light have been modified to eliminate the need forthe focusing lens(es) 84 of FIG. 5, thereby shortening the device. Laserdiodes have a very astigmatic output pattern and diverge rapidly(typically 35-45° full-width at half-maximum (FWHM)) perpendicular tothe thin diode stripe on the diode output (front) face and more slowlyin the direction parallel to the stripe (typically 8-12° FWHM). Thediode stripe is typically 1 μm thick and 50-300 μm wide for a multimodedevice. In the embodiment of FIG. 6, a laser diode 120 mounted on a heatsink 122 has a collimating lens 124 attached to it close to the diodeoutput face. The lens 124 may be collimating in both parallel andperpendicular directions and can produce an output beam that iscollimated or with nearly equal divergence in both directions. For thedesign shown in FIG. 6 the diode light is collimated in both directionsand the diode output beam profile is typically close to a square. Thediode assembly 116 and collimating lens 124 are placed as close aspossible to the vanadate laser crystal 72. By eliminating focusingoptics the configuration shown in FIG. 6 is more compact than that ofFIG. 5. In addition lens losses are eliminated. This increase inefficiency is somewhat mitigated however by some of the diode excitationbeing lost since the extracting beam is round, not square.

[0061] In the embodiment shown in FIG. 7, the outcoupler of FIG. 6 hasbeen eliminated by placing the curvature required to make the resonatorstable and mode match the internal mode to the diode spot diameter, onthe nonlinear harmonic generation crystal 126. As shown in FIG. 7, thefront face 128 of the nonlinear harmonic generation crystal 126 has aconvex-shape to provide the proper curvature. The harmonic highreflector 78 can also be eliminated in the embodiments shown in FIGS.5-7 by placing a coating 94, which is HR for both the fundamental andharmonic wavelengths, directly on the rear face 92 of the laser crystal72. This configuration is the simplest and occupies the least amount oflinear distance of any of the embodiments discussed thus far.

[0062]FIG. 8 illustrates a fourth embodiment of a green light solidstate laser and diode for use in laser flashlight 10 in which thesolid-state laser resonator has been eliminated altogether. The outputof the diode 120 is collimated by passage through a collimating lens124. The astigmatic nature of the laser light is eliminated by lettingthe collimated output propagate through a length of multimode opticalfiber 134 where the output is completely homogenized and circular due tothe mixing and excitation of numerous propagation modes in the fiber134. If the numerical aperture of the fiber 134 is large enough thediode light is simply allowed to diverge where it is collimated and madeeye-safe using an objective lens 112. If the numerical aperture is notlarge enough, however, a GRIN or other lens 114 can be inserted toincrease the divergence. The optical fiber 134 may also be eliminated ifoutput homogeneity is not that important. The objective lens 112 thenwill also act as an aperture and produce a circular output beam at theexpense of laser efficiency since part of the square output beam fromthe collimating lens 124 will be lost.

[0063] An alternative to using the diode package as described above isto utilize a plurality of diode packages of either the same or differentfrequencies. This alternative provides for the possibility of producinglaser beams of multiple frequencies simultaneously. With reference toFIG. 9, a flashlight 10″ in accordance with the invention may produce adual wavelength laser output in the visible region. A frequency-doubled,green 532 nm Nd:YVO₄ laser 136 is combined with a red diode laser 138operating in the 630-670 nm spectral range. An integrated power supply14 may provide excitation to either laser separately or at the sametime. Combining the output from both lasers 136, 138 is achieved with apolarizer 140. Preferably, polarizer 140 transmits all of the green 532nm light for “s” polarization (in the plane of the Figure) and reflectsall of the red 630-670 nm light which has “p” polarization(perpendicular to the plane of the Figure). Alternatively, polarizer 140may transmit all of the green 532 nm light for “p” polarization andreflect all of the red 630-670 nm light which has “s” polarization. Beamforming optics 142, 144, which comprise various reflective or refractiveoptics, are used to manipulate the output of both lasers 136, 138,providing the proper beam shape, size and divergence so that both may bearranged to enter the first beam expander lens 146. The first and secondbeam expander lenses 146, 148 expand both the coincident and parallelbeams and transmit a laser beam from the enclosure. The design of theflashlight 10″ must take into account the effect of the differentrefractive power of optical elements such as lenses 142, 144 for the twodifferent wavelengths of laser light.

[0064] In embodiments employing a plurality of diode-pumped lasers, thevarious diodes may be operated in a continuous wave mode and/or in amodulated mode, i.e., the diodes could be operated in either continuouswave or modulated modes independently of one another. As yet anotheralternative, a diode package can be operated to generate laser beams ata plurality of wavelengths.

[0065] While the present invention has been described in connection withwhat is presently considered to be the most practical and preferredembodiments, it is understood that the invention is not limited to thedisclosed embodiments.

What is claimed is:
 1. A hand-held laser flashlight comprising: ahousing having a light emission surface; a coherent light sourcedisposed within the housing, said coherent light source emitting acoherent light beam; a power supply coupled to said coherent lightsource; an optical system disposed within said housing, said opticalsystem having an optical system input and an optical system output,wherein said coherent light beam enters through said optical systeminput, and wherein a laser beam with a substantially gaussian spatialprofile exits through said optical system output; and a beam expanderdisposed within said housing, said beam expander having a beam expanderinput and a beam expander output, wherein said laser beam enters throughsaid beam expander input, and wherein an intensity controlled light beamexits through said beam expander output and through said light emissionsurface, said intensity controlled light beam having a beam intensity ofless than 26 mW/cm².
 2. A hand-held laser flashlight comprising: ahousing having a light emission face; a diode light source disposedwithin the housing, said diode light source emitting a coherent lightbeam toward said light emission face along an optical axis; a powersupply coupled to said diode light source; an optical system disposedwithin the housing along the optical axis intermediate said diode lightsource and said light emission face, said optical system outputting alaser beam toward said light emission face with a substantially gaussianspatial profile; and a light transmissive beam expander disposed alongthe optical axis intermediate said optical system and said lightemission face, the laser beam traversing said light transmissive beamexpander and exiting an exit surface of said light transmissive beamexpander as an output laser beam, said light transmissive beam expanderconfigured to limit a beam intensity corresponding to said output laserbeam to less than 26 mW/cm².
 3. The laser flashlight of claim 2, whereinthe diode light source is comprised of a plurality of laser diodes. 4.The laser flashlight of claim 2, wherein the optical system furthercomprises a resonator, said resonator comprised of an active gainelement pumped by the diode light source.
 5. The laser flashlight ofclaim 4, said resonator further comprising a harmonic generatingcrystal, the active gain element receiving the coherent light beamemitted by the diode light source and emitting an intermediate laserbeam having an intermediate wavelength, wherein said harmonic generatingcrystal shifts the intermediate wavelength to an output wavelengthcorresponding to the output laser beam.
 6. The laser flashlight of claim4, further comprising: an optically transmissive heat conductive elementin thermal contact with said active gain element; and a heat sink inthermal contact with said optically transmissive heat conductiveelement.
 7. The laser flashlight of claim 6, wherein the opticallytransmissive heat conductive element has opposed first and second endsurfaces, said first and second end surfaces coated with ananti-reflection coating.
 8. The laser flashlight of claim 5, wherein theactive gain element is a laser crystal having opposed first and secondaxially spaced end surfaces, the first end surface of the laser crystalfacing the diode light source and coated for high transmission at awavelength corresponding to the coherent light beam and for highreflectivity at the intermediate and output wavelengths.
 9. The laserflashlight of claim 5, wherein the resonator further includes an outputcoupler having an input end surface coated for high reflectivity at theintermediate wavelength and for high transmission at the outputwavelength.
 10. The laser flashlight of claim 4, wherein the resonatorfurther includes an output coupler having an input end surface coatedfor partial reflectivity at the output laser beam wavelength.
 11. Thelaser flashlight of claim 5, wherein the resonator further comprises anoptically transmissive harmonic mirror element axially disposedintermediate the active gain element and the harmonic generatingcrystal, the harmonic mirror having opposed first and second axiallyspaced end surfaces, the second end surface of the harmonic mirrorfacing the harmonic generating crystal and being coated for hightransmission at the intermediate wavelength and for high reflectivity atthe output wavelength.
 12. The laser flashlight of claim 5, wherein theharmonic generating crystal has an end surface facing the active gainelement, the end surface of the harmonic generating crystal being coatedfor high transmission at the intermediate wavelength and for highreflectivity at the output wavelength.
 13. The laser flashlight of claim5, wherein the harmonic generating crystal has a substantially flatfirst end surface and a convex-shaped second end surface, the first endsurface facing the active gain element.
 14. The laser flashlight ofclaim 2, wherein the light transmissive beam expander further comprisesat least one collimating lens and a gradient index or aspheric lensdisposed along the optical axis.
 15. The laser flashlight of claim 3,wherein the diode laser source further comprises an optical fiberdisposed intermediate the diode laser source and the optical system. 16.The laser flashlight of claim 2, wherein the diode laser source furthercomprises a diode array and a collimating lens for receiving,collimating and transmitting the coherent light beam emitted by thediode laser source.
 17. The laser flashlight of claim 2, wherein theoptical system further comprises an optical fiber having a first end forreceiving the coherent light beam emitted by the diode laser source anda second end for transmitting the coherent light beam to the lighttransmissive beam expander.
 18. The laser flashlight of claim 17,wherein the light transmissive beam expander comprises a gradient indexor aspheric lens.
 19. The laser flashlight of claim 2, furthercomprising means for pulsing the output laser beam.
 20. The laserflashlight of claim 2, wherein the output laser beam has a spatialprofile that is substantially TEM₀₀ mode.
 21. A laser flashlightcomprising: a housing having a light emitting end; an emitter disposedwithin the housing emitting a coherent light toward the light emittingend of the housing along an optical axis upon the application ofelectricity; a power supply for selectively applying electricity to theemitter; an optical resonator disposed within the housing and along theoptical axis, the optical resonator intermediate the emitter and thelight emitting end of the housing, the optical resonator comprising: alaser element pumped by the emitter; a frequency converter; and couplingoptics, the optical resonator producing an output laser beam witha-spatial profile that is substantially TEM₀₀ mode; and a lighttransmissive beam expander disposed proximate to the light emitting endof the housing and along the optical axis, the light transmissive beamexpander receiving and dispersing the output laser beam, wherein saidlight transmissive beam expander is configured to limit a beam intensitycorresponding to said output laser beam to less than 26 mW/cm².
 22. Thelaser flashlight of claim 21, wherein the frequency converter comprisesat least one harmonic generating crystal.
 23. A laser flashlightcomprising: housing means, said housing means having a light emittingend; means disposed within the housing means for emitting a coherentlight toward the light emitting end of the housing means along anoptical axis; means for selectively applying electricity to the coherentlight emitting means; means disposed within the housing means forreceiving the coherent light and emitting an output laser beam with aspatial profile that is substantially TEM₀₀ mode; and means disposedproximate to the light emitting end for receiving and dispersing theoutput laser beam, said receiving and dispersing means configured tolimit an intensity corresponding to said output laser beam to less than26 mW/cm².
 24. The laser flashlight of claim 23, further comprisingmeans for pulsing the output laser beam.
 25. The laser flashlight ofclaim 24, wherein the means for pulsing the output laser beamselectively interrupts the output laser beam to produce intermittentasymmetric laser beam pulses.